Chapter 6 Looping CS185/09 - Introduction to Programming Caldwell College

2 Chapter 6 Topics While Statement Syntax Count-Controlled Loops Event-Controlled Loops Using the End-of-File Condition to Control Input Data Using a While Statement for Summing and Counting Nested While Loops Loop Testing and Debugging

3 What is a loop? A loop is a repetition control structure It causes a single statement or block to be executed repeatedly

4 Two Types of Loops Count controlled loops –repeat a specified number of times Event controlled loops –some condition within the loop body changes and this causes the repeating to stop

5 While Statement SYNTAX while ( Expression ) {.. // loop body. } NOTE: Loop body can be a single statement, a null statement, or a block The while loop is considered a pre-test or top-tested loop

6 When the expression is tested and found to be false, the loop is exited and control passes to the statement which follows the loop body. WHILE LOOP FALSE TRUE body statement Expression

7 Count-controlled loop contains An initialization of the loop control variable An expression to test for continuing the loop An update of the loop control variable to be executed with each iteration of the body

8 Count-controlled Loop int count; count = 4; // initialize loop variable while (count > 0) // test expression { cout << count << endl; // repeated action count--; // update loop variable } cout << “Done” << endl;

9 Count-controlled Loop int count; count = 4; while (count > 0) { cout << count << endl; count--; } cout << “Done” << endl; OUTPUT Done

10 Count-Controlled Loop Example myInfile contains 100 blood pressures Use a while loop to read the 100 blood pressures and find their total

11 Count-Controlled Loop Example ifstream myInfile; int thisBP; int total; int count; count = 0; // initialize while ( count < 100 ) // test expression { myInfile >> thisBP; total = total + thisBP; count++; // update } cout << “The total = “ << total << endl;

12 Event-controlled Loops Sentinel controlled –keep processing data until a special value which is not a possible data value is entered to indicate that processing should stop End-of-file controlled –keep processing data as long as there is more data in the file Flag controlled –keep processing data until the value of a flag changes in the loop body

13 Examples of Kinds of Loops Count controlled loop –Read exactly 100 blood pressures from a file End-of-file controlled loop –Read all the blood pressures from a file no matter how many are there Sentinel controlled loop –Read blood pressures until a special value (like -1) selected by you is read Flag controlled loop –Read blood pressures until a dangerously high BP (200 or more) is read

14 Sentinel controlled loop Requires a “priming read” –A “priming read” means you read one set of data before the while Exits the loop when the sentinel value is encountered

15 Sentinel controlled loop total = 0; cout << “Enter a blood pressure (-1 to stop ) ”; cin >> thisBP; while (thisBP != -1)// while not sentinel { total = total + thisBP; cout << “Enter a blood pressure (-1 to stop ) ”; cin >> thisBP; } cout << total;

16 End-of-File Controlled Loop Depends on fact that a file goes into fail state when you try to read a data value beyond the end of the file The program often needs special processing when the final input record has been processed –Close the input file –Produce a summary report –Etc.

17 End-of-File Controlled Loop total = 0; myInfile >> thisBP; // priming read while (myInfile) // while last read successful { total = total + thisBP; myInfile >> thisBP; // read another } cout << total;

18 End-of-File at keyboard total = 0; cout << “Enter blood pressure (Ctrl-Z to stop)”; cin >> thisBP; // priming read while (cin) // while last read successful { total = total + thisBP; cout << “Enter blood pressure”; cin >> thisBP; // read another } cout << total;

19 Flag-controlled Loops You initialize a flag (to true or false) Use meaningful name for the flag A condition in the loop body changes the value of the flag Test for the flag in the loop test expression

20 Flag-controlled Loops countGoodReadings = 0; isSafe = true; // initialize Boolean flag while (isSafe) { cin >> thisBP; if ( thisBP >= 200 ) isSafe = false; // change flag value else countGoodReadings++; } cout << countGoodReadings << endl;

21 Loops often used to Count all data values Count special data values Sum data values Keep track of previous and current values

22 Loop Testing and Debugging Test data should test all sections of program Beware of infinite loops -- program doesn’t stop Check loop termination condition, and watch for “off-by- 1” problem Use get function for loops controlled by detection of ‘\n’ character Use algorithm walk-through to verify pre- and post- conditions Trace execution of loop by hand with code walk-through Use a debugger to run program in “slow motion” or use debug output statements